1. Field of the Invention
The present invention relates to an image pickup device comprising an image pickup element and a lens for focusing an image onto the imaging face of the image pickup element.
2. Description of the Related Art
Recently, an image pickup device comprising a solid-state image pickup element and a lens for focusing an image onto the imaging face thereof has been applied to a camera. An optical lens having uniform refractive index is used as a lens for such an image pickup device as described above.
In the image pickup device using the lens having uniform refractive index, an optical distance which is extremely large although it is finite is required between the lens and the solid-state image pickup element in order to converge and focus image or refracted light beams onto the imaging face of the solid-state image pickup element. Accordingly, the sum of the thickness of the lens and the imaging distance is needed as the A optical distance between a first face (i.e., the face at the light incident side) of the lens and the imaging face of the solid-state image pickup element.
In the conventional image pickup device as described above, since the total of the lens thickness and the imaging distance is needed as the optical distance between the lens first face (i.e., the incident-side face) and the imaging face of the solid-state image pickup element, and thus it is very difficult to shorten the overall length of the optical system. Therefore, requirements of compact design for the image pickup device and the camera using it have never been satisfied.
According to the above-described conventional image pickup device, when a lens holder is used to secure a lens so as to be spaced from the image pickup element at a proper interval while a predetermined positional relationship is kept between the lens and the image pickup element, an error of the positional relationship between the lens and the lens holder and an error of the positional relationship between the lens holder and the solid-state image pickup element may occur as a fabrication error, and thus it is difficult to reduce the error.
The present invention has been implemented to overcome the problem of the conventional image pickup device, and has an object to more shorten the overall length of an image pickup device comprising a lens and an image pickup element, and reduce the error of the positional relationship between the lens and the image pickup element.
In order to attain the above object, an image pickup device according to the present invention is characterized in that a refractive index distribution lens having a refractive index distribution which is inversely proportional to the square of the distance from the optical axis in a cross-section vertical to the optical axis is provided as an imaging lens in the neighborhood of the imaging face of the image pickup element.
According to the image pickup device of the present invention, since the refractive index lens is used as the lens, an image at infinity which is incident as parallel rays of light from the end face of the lens incident side is imaged on the end face of the lens emission side when the length (thickness) of the lens is set to a meandering period P=0.5xcfx80+nxcfx80 (n=0, 1, 2, . . . ). Accordingly, the image is imaged on the imaging face of the image pickup element located in the neighborhood of the end face of the lens emission side, and thus it is unnecessary to set a large gap between the lens and the image pickup element. Therefore, the length of the optical system can be reduced, and the design of the image pickup device can be improved to be more compact in size and thinner in thickness.
Further, the lens can be provided in the neighborhood of the image pickup element, and thus the positional relationship between the image pickup element and the lens can be fixed by adhesion with organic solvent or the like, whereby an error factor causing the positional error of the lens to the image pickup element can be reduced and thus the positioning precision can be enhanced.
The image pickup device of the present invention is basically characterized in that a refractive index distribution lens having a refractive index distribution which is substantially proportional to the square of the distance from the optical axis in cross-section vertical to the optical axis is provided as an imaging lens in the neighborhood of the imaging face of an image pickup element. The lens length is preferably set to a meandering period P=0.5xcfx80+nxcfx80 (n=0, 1, 2, . . . ). Particularly, it is preferable that the lens length is as small as possible (for example, n=zero) because the lens length can be shortened.
The image pickup device of the present invention can be embodied in such a manner that the refractive index distribution lens and the image pickup element are held by a holder to regulate the positional relationship between the refractive index distribution lens and the image pickup element. However, it may be embodied in such a manner that the refractive index distribution lens is directly adhesively attached to the image pickup element. In this case, the positional relationship between the refractive index distribution lens and the image pickup element can be controlled with extremely high precision if a material which gives no adverse effect to the lens and the image pickup element is merely selected as the adhesive agent.
Further, an optical thin film for reflecting infrared rays may be provided at the light incident face of the refractive index distribution lens, or absorption means for absorbing infrared rays may be provided at the light incident face side of the refractive index distribution lens. With this structure, the infrared rays can be intercepted.
Further, a lens which is provided with a curvature at one end face side or both the end face sides thereof may be used as the refractive index distribution lens. In this case, the optical characteristic based on the refractive index distribution is varied by the curvature, and the combined optical characteristic of the optical characteristic based on the refractive index distribution and the optical characteristic based on the curvature becomes the optical characteristic of the lens. As described above, the present invention has various modes.
The xe2x80x9cimage pickup elementxe2x80x9d of the present invention means any member which can receive an image of rays of light from a subject and output the electrical signal corresponding to the image, such as a CCD (charge-coupled device), a CMOS image sensor or the like.